Experimental study on the production of Ac-225 using an electron linear accelerator

Objectives: Ac-225 has nuclear properties well suited for use in targeted alpha therapy and clinical trials have demonstrated the applicability of radiopharmaceuticals containing Ac-225 to treat various cancers. Ac-225 is mainly obtained by radiochemical separation from Th-229 sources, but supply shortages are expected when Ac-225 is generally applied. Thus accelerator-based Ac-225 production systems are desired. In this work, we have carried out an experimental study on the production of Ac-225 using an electron linear accelerator. Methods: We carried out the Ac-225 production test using the electron linear accelerator of Research Center for Electron Photon Science, Tohoku University with three electron beam energies: 33.3MeV, 38.9MeV and 44.4MeV. We used Ra-226•Cl₂ (Ra-226, 50kBq) as a target nuclide material. Ac-225 is in radiation equilibrium with Bi-213, hence we evaluated Ac-225 production rate using gamma-spectrometry of Bi-213 with a HP-Ge detector. We also derived a production rate of Ra-225 using calculated bremsstrahlung spectra and a theoretical reaction cross section data for the Ra-226(γ,n)Ra-225 nuclear reaction to compare the experimental values. Ac-225 production rate was derived by calculation of radiation balance with Ra-225.Results: We found that experimental Ac-225 production amounts per unit irradiation current at electron beam energies of 33.3MeV, 38.9MeV and 44.4MeV were 0.28 Bq/µA/h, 0.36 Bq/µA/h and 0.40Bq/µA/h. These values were 1.49, 1.39 and 1.12 times larger than the calculated value.

Conclusions: The production amount of Ac-225 increased with the increase in electron beam energy, but comparison of the measured value to the evaluated value by the simulation showed it to be smaller as the electron beam energy was increased.